Hydraulic systems are employed in many circumstances to provide hydraulic power from a hydraulic power source to multiple loads. In particular, such hydraulic systems are commonly employed in a variety of work vehicles such as excavators and loader-backhoes. In such vehicles, the loads powered by the hydraulic systems may include a variety of actuatable devices such as cylinders that lower, raise and rotate arms, and lower and raise buckets, as well as hydraulically-powered motors that drive tracks or wheels of the vehicles. Although the various actuatable devices typically are powered by a single source (e.g., a single pump), the rates of fluid flow to the different devices typically are independently controllable, through the use of separate control valves (typically spool valves) that are independently controlled by an operator of the work vehicle.
The operation of the actuatable devices depends upon the hydraulic fluid flow to those devices, which in turn depends upon the cross-sectional areas of metering orifices of the control valves between the pressure source and the actuatable devices, and also upon the pressure differentials across those metering orifices. To facilitate control, hydraulic systems often are pressure compensated, that is, designed to set and maintain the pressure differentials across the metering orifices of the control valves, so that controlling of the valves by an operator only tends to vary the cross-sectional areas of the orifices of those valves but not the pressure differentials across those orifices. Such pressure compensated hydraulic systems typically include compensation valves positioned between the respective control valves and the respective actuatable devices. The compensation valves control the pressures existing on the downstream sides of the metering orifices to produce the desired pressure differentials across the metering orifices.
Such pressure compensated hydraulic systems normally ensure that the same particular pressure differential (e.g., a pump margin pressure) occurs across each of the control valves. Nevertheless, it is desirable in some hydraulic systems to have a lower pressure differential across selected valves to reduce the hydraulic fluid flow through those valves. For example, in the case of an excavator, it may be desirable to provide normal hydraulic fluid flow to the cylinders that control lifting or other movement of an arm or bucket of the excavator, or to accessories of the excavator such as a trenching device, yet at the same time desirable to provide reduced hydraulic fluid flow to the hydraulic motors controlling the speeds of the tracks of the excavator so that the excavator travels at reduced speeds. Therefore, there is a need in some hydraulic systems to provide a pressure differential across metering orifices in selected control valves which is less than the pressure differential across other control valves.
Various modifications to pressure compensated hydraulic systems have been developed in the past to allow for different pressure differentials across different control valves. One modification is to place an additional orifice in series with the control valve, where the additional orifice may be fixed to define the maximum flow or it may be adjustable so that the operator can select a desired flow. Another technique, with a spring-operated compensation valve, is to adjust the spring load mechanically while leaving the metering area constant. Both of these conventional techniques require additional mechanical devices that may be difficult to implement or locate with respect to existing valve components in a valve assembly. The latter technique also requires sizeable springs to handle the relatively large loads that act on them.
Further, using these conventional techniques, it is difficult or impossible to adjustably control the pressure differentials across multiple control valves so that each of the control valves experiences the same pressure differential. In particular, the providing of fixed additional orifices does not allow for adjustable control of pressure differentials, while the providing of individual adjustment springs for each compensation valve makes it difficult for an operator to evenly set the pressure differentials occurring across different control valves.
This capability of providing adjustable control of the pressure differentials across multiple control valves in an even manner is nevertheless desirable in many circumstances, since it is often desirable that multiple hydraulic devices of a hydraulic system should receive precisely identical amounts of hydraulic fluid flow when an operator sets the respective control valves identically. For example, with respect to the excavator discussed above, it would be desirable that the hydraulic motors corresponding to the left and right tracks of the excavator be driven at the exact same speed assuming that the operator of the excavator set the control valves for those motors to the same level.
Therefore, it would be advantageous if pressure compensated hydraulic systems could be designed so that reduced pressure differentials could be imparted across multiple control valves without the use of many additional, unwieldy components. Additionally, it would be advantageous if pressure compensated hydraulic systems could be designed to allow for adjustable control of the pressure differentials across multiple control valves, where the adjustments affected each of the pressure differentials equally. It would further be advantageous if such modified pressure compensated hydraulic systems allowed for an operator to adjust the pressure differentials across multiple control valves by way of a single switch and/or dial that imparted desired adjustments to all of the multiple control valves simultaneously. Additionally, it would be advantageous if such pressure compensated hydraulic systems allowing for adjustable control did not require significant additional numbers of components, and were otherwise relatively inexpensive to implement, in comparison with existing pressure compensated hydraulic systems.